There are many potential applications for devices fabricated from semiconducting diamond operating in a wide variety of extreme environmental conditions, at high frequencies, and in high power applications. Such wide use is possible primarily because of the many unique properties of diamond such as chemical inertness, high electron and hole mobilities, low dielectric constant, radiation hardness, mechanical strength, large electron energy band gap, and optical transparency.
One such device structure, the diamond field effect transistor (FET) has been proposed and fabricated on homoepitaxial layers deposited by chemical vapor deposition, single crystal substrates, and polycrystalline thin films also deposited by CVD.
The design and fabrication of horizontal diamond FETs have been widely reported in the art. See for example, U.S. Pat. No. 3,603,848 entitled Complementary Field-Effect-Type Semiconductor Device by Sato et al. and publications entitled High-Temperature Thin-Film Diamond Field-Effect Transistor Fabricated Using a Selective Growth Method by Gildenblat et al., IEEE Electron Device Letters, Vol. 12, No. 2, pp. 37-39 (Feb. 1991); Fabrication of an Insulated Gate Diamond FET for High Temperature Applications by Hewett et al., presented at the International High Temperature Electronics Conference in Albuquerque, N.M., pp. 168-173 (Jun. 1991); and IGFET Fabrication of Homoepitaxial Diamond Using in Situ Boron and Lithium Doping by Fountain et al., presented at the Electrochemical Society meeting held in Washington, D.C. (May 1991). See also the publication by Tessmer et al., Polycrystalline diamond field-effect transistors, Diamond and Related Materials I, Elsevier Science Publishers B. V., Amsterdam, Holland, pp. 89-92 (1992).
The active channels of the devices are formed from semiconducting diamond using either naturally occurring type IIb single crystals, in-situ boron doped homoepitaxial layers or polycrystalline diamond deposited in silicon substrates, ion implantation into single crystal diamond, or through use of an elevated temperature diffusion process. Some of these devices employ a selected area deposition of homoepitaxial diamond films, with in-situ boron doping, grown on insulating crystals.
In some cases, a metal-semiconductor (MS) junction formed by depositing a metal contact directly on the semiconducting diamond film was employed to form a so-called "depletion-mode diamond metal-semiconductor field effect transistor" (MESFET) such as described in Tsai et al. Diamond MESFET Using Ultrashallow RTP Boron Doping Electron Device Letters, Vol. 12, No. 4, pp. 157-159 (1991). By choosing a suitable metal, it was possible to provide a junction displaying rectifying characteristics provided semiconducting diamond films of low dopant concentrations were used. If a diamond film of high dopant concentration was employed, it became difficult to maintain a low leakage rectifying contact. Furthermore, even for films of low dopant concentrations, it was impossible to use the MESFET in enhancement mode because current tended to flow through the MS junction when even small forward biases were applied to the gate contact, and significantly decreased breakdown voltages provided for poor operation at elevated temperatures even when operated in depletion mode.
Some devices employed a junction incorporating a film of silicon dioxide between the metal and the semiconducting diamond to form a depletion-mode metal oxide semiconductor FET (MOSFET), such as described, for example, in Grot et al., The Effect of Surface Treatment on the Electrical Properties of Metal Contacts to Boron-Doped Homoepitaxial Diamond Film, IEEE Elect. Device Letters, Vol. 11, No. 2, pp. 100-102 (Feb. 1990), and Tessmer et al., Polycrystalline diamond field-effect transistors, Diamond and Related Materials I, pp. 89-92 (1992). Other devices employed a junction including an intrinsic diamond film to form a metal-intrinsic diamond-semiconductor FET (MiSFET) such as described in U.S. Pat. No. 5,173,761 entitled Semiconducting Polycrystalline Diamond Electronic Devices Employing An Insulating Diamond Layer. In the case of the MiSFET, inevitable defects in nominally undoped diamond films and contamination of the nominally undoped diamond films through the action of auto-doping effects of the underlying doped diamond layer meant that operation in enhancement mode was extremely limited even at low temperatures and depletion mode performance was deteriorated at elevated temperatures.
High temperature operation is also limited for a MOSFET device. The possibility of surface reactions and a high field region at the diamond/silicon dioxide interface can cause decreased breakdown voltages and large leakage currents at elevated temperatures. If operated in enhancement mode, the SiO2/diamond interface becomes critical to the device performance as the majority of current transport occurs at the interface and consequently the action of the above effects means that enhancement mode operation particularly at elevated temperatures is severely limited.
The diamond film FET devices of the prior art currently exhibit poor leakage characteristics, especially at elevated temperatures where diamond electronic devices are expected to have a significant advantage over other materials. There is at present a demand for a junction suitable for use in devices such as FETs through which the excellent characteristics of diamond at high temperatures may be more fully exploited.